Remove iron workflows and update readme (#612)

Author: christophfroehlich

.github/dependabot.yml

@@ -11,13 +11,6 @@ updates:
     directory: "/"
     schedule:
       interval: "weekly"
-  - package-ecosystem: "github-actions"
-    # Workflow files stored in the
-    # default location of `.github/workflows`
-    directory: "/"
-    schedule:
-      interval: "weekly"
-    target-branch: "iron"
   - package-ecosystem: "github-actions"
     # Workflow files stored in the
     # default location of `.github/workflows`

.github/mergify.yml

@@ -8,15 +8,6 @@ pull_request_rules:
         branches:
           - humble
 
-  - name: Backport to iron at reviewers discretion
-    conditions:
-      - base=master
-      - "label=backport-iron"
-    actions:
-      backport:
-        branches:
-          - iron
-
   - name: Ask to resolve conflict
     conditions:
       - conflict

.github/workflows/iron-binary-build.yml

@@ -112,7 +112,6 @@ ROS 2 Distro | Branch | Build status | Documentation
 :----------: | :----: | :----------: | :-----------:
 **Rolling** | [`master`](https://github.com/ros-controls/ros2_control_demos/tree/master) | [![Rolling Binary Build](https://github.com/ros-controls/ros2_control_demos/actions/workflows/rolling-binary-build.yml/badge.svg?branch=master)](https://github.com/ros-controls/ros2_control_demos/actions/workflows/rolling-binary-build.yml?branch=master) <br /> [![Rolling Semi-Binary Build](https://github.com/ros-controls/ros2_control_demos/actions/workflows/rolling-semi-binary-build.yml/badge.svg?branch=master)](https://github.com/ros-controls/ros2_control_demos/actions/workflows/rolling-semi-binary-build.yml?branch=master) <br /> | [Documentation](https://control.ros.org/master/index.html) <br /> [API Reference](https://control.ros.org/master/doc/api/index.html)
 **Jazzy** | [`master`](https://github.com/ros-controls/ros2_control_demos/tree/master) | [![Rolling Binary Build](https://github.com/ros-controls/ros2_control_demos/actions/workflows/rolling-binary-build.yml/badge.svg?branch=master)](https://github.com/ros-controls/ros2_control_demos/actions/workflows/rolling-binary-build.yml?branch=master) <br /> [![Rolling Semi-Binary Build](https://github.com/ros-controls/ros2_control_demos/actions/workflows/rolling-semi-binary-build.yml/badge.svg?branch=master)](https://github.com/ros-controls/ros2_control_demos/actions/workflows/rolling-semi-binary-build.yml?branch=master) <br /> | [Documentation](https://control.ros.org/master/index.html) <br /> [API Reference](https://control.ros.org/jazzy/doc/api/index.html)
-**Iron** | [`iron`](https://github.com/ros-controls/ros2_control_demos/tree/iron) | [![Iron Binary Build](https://github.com/ros-controls/ros2_control_demos/actions/workflows/iron-binary-build.yml/badge.svg?branch=iron)](https://github.com/ros-controls/ros2_control_demos/actions/workflows/iron-binary-build.yml?branch=iron) <br /> [![Iron Semi-Binary Build](https://github.com/ros-controls/ros2_control_demos/actions/workflows/iron-semi-binary-build.yml/badge.svg?branch=iron)](https://github.com/ros-controls/ros2_control_demos/actions/workflows/iron-semi-binary-build.yml?branch=iron) <br /> | [Documentation](https://control.ros.org/iron/index.html) <br /> [API Reference](https://control.ros.org/iron/doc/api/index.html)
 **Humble** | [`humble`](https://github.com/ros-controls/ros2_control_demos/tree/humble) | [![Humble Binary Build](https://github.com/ros-controls/ros2_control_demos/actions/workflows/humble-binary-build.yml/badge.svg?branch=humble)](https://github.com/ros-controls/ros2_control_demos/actions/workflows/humble-binary-build.yml?branch=humble) <br /> [![Humble Semi-Binary Build](https://github.com/ros-controls/ros2_control_demos/actions/workflows/humble-semi-binary-build.yml/badge.svg?branch=humble)](https://github.com/ros-controls/ros2_control_demos/actions/workflows/humble-semi-binary-build.yml?branch=humble) <br /> | [Documentation](https://control.ros.org/humble/index.html) <br />[API Reference](https://control.ros.org/humble/doc/api/index.html)